New Links, Programming Instructions, and the Rationale of the One-fire Firing Schedule

It has come to my attention that some of you have been continuing to look for the instruction manuals, which I had removed from the right panel of this blog. All the instruction manuals can now be found directly on the product pages of my online store — the page for each Hadar’s Clay product includes a link to the instruction manual that applies to that product. For your convenience, though, I have now re-linked them from the right panel of the blog.

A new document has been uploaded and is linked here as well. It contains programming instructions for the One-fire Clays (including One-fire Flex Clays). The instructions are step-by-step, with photos. On the right side of each photo you will find an explanation of what that step means. At the end of the document is the full firing schedule for all the One-fire Clays in a single page, so you can print it out separately.

For those of you who are new to the One-fire Clays and to metal clay in general, I’d like to explain the rationale behind the firing schedule.

First Ramp

The kiln ramps at 1800°F. That means that it heats at a speed of 1800 degrees (Fahrenheit) per hour. (Bear in mind, it will not reach 1800 degrees; this is just the speed at which it will reach the desired temperature). In many kilns, 1800°F is the full speed.

First Hold temperature

When the kiln reaches 1000°F in a brick kiln, or 1100°F in a muffle kiln, it needs to stay at this temperature for a while. This is the temperature at which the binder burns, and it is the same for all clays! If the kiln were to keep heating rather than holding at this temperature, the binder would not be able to burn anymore because the reduction process would already have already began.

Reduction? That means that as the temperature rises above 1000°F (brick)/1100°F (muffle), less oxygen is now present in the kiln. With not enough oxygen, the binder cannot burn off. If the binder has not burned off yet, it will not burn at all, and that will end in poor sintering results. Failure to burn the binder is the most common cause for poor sintering. Some metals, bronze for example, will sinter well and be strong even if their final firing temperature is much lower than prescribed, but only if the binder has burnt off completely.

First Hold time

Hold time depends on how much binder you need to burn. The more pieces present in the kiln, or the larger they are, the longer you need to hold. Personally, I never had to hold more than two hours, and I do fire large pieces. For one small piece I would hold 0:30 minutes. For an average piece or just 2-3 pieces I would hold 1:00 hour.

Second Ramp

This ramp speed is the same as the first one. All we need now is to get to the sintering temperature.

Second hold Hold temperature

Now that that the binder is gone, the clay starts sintering. That means that the particles of the metal powder are getting closer and closer to each other until they cannot get any closer. This temperature is different from one clay to another and for each combination of clays. For some clays, like copper, if they are fired at a lower temperature than prescribed, they may still sinter, but will not be as strong as they can be. You can read more about this process in my blog posting entitled “The Sintering Project“.

Second Hold time

The second hold time is always 2:00 hours.


Why do clays other than One-fire (like Quick-fire) not require hold time? The reason is that these clays are fired twice anyway, with a cooling phase between firings. That gives the binder enough time to burn off. These clays, by the way, will not sinter without cooling between the two firings, no matter how long you hold at 1000°F (brick)/1100°F (muffle).

Why is the the temperature different for brick and muffle kilns? Muffle kilns (usually front loading, white walls, with hidden heating elements), fire 50°F lower than brick kilns, probably due to a difference in heat distribution. In those kilns it is also recommended to elevate the firing vessel as far as an inch from the top of the chamber. It this spot the actual temperature in the kiln is the closest to what the controller displays.

Why not ramp slower than 1800°F per hour instead of holding at 1000°F (brick)/1100°F (muffle)? The binder needs a certain amount of time at 1000°F (brick)/1100°F (muffle). If you slow-ramp, you give it more time at temperatures lower than 1000°F (brick)/1100°F (muffle), a temperature not high enough for it to burn off, and very little time at 1000/1100°F, the temperature it actually needs.

I hope this shed some light on the process. Please feel free to ask for clarifications.

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